Stellar Classification

In astronomy, stellar classification is a way of grouping stars by temperature and luminosity. A staris a ball of superheated gas called plasma. Star temperature can be measured by looking at its spectrum, the type of light that the star shines. Stars are also grouped into spectral types or classes by color. In general, a star's temperature determines its color, from red to blue-white. Blue-white stars are the hottest, while Red ones are the coolest. Each class has their own subclass as well, with Hindu-Arabic Numerals. A G0 star would be the hottest type G and G9 with be the coolest type G.

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Life Cycle of a Star

Stellar life cycles.

Stars are born in clouds of gas called a Nebula (plural: Nebulae) Nebulae are pulled together and become Protostars. Clouds of gas orbiting these stars then condense to form Planets, like how the Solar System was formed. The Star is then born and becomes a Main-Sequence Star. These stars live for up to 10 billion years and start to fuse hydrogen into the other elements of the Periodic Table. But as they reach heavier elements like Oxygen, they start cooling in temperature and become Red Giant stars, pushing their outer layers outward. Some stars expand even further into Red Supergiants by fusing metals. If the original post-giant star is more than 7 times larger than the Sun, then it will explode as a supernova for its death. Then it will become either a Neutron Star or a Black Hole, a bottomless pit that nothing can escape. If the original post-giant star is less than 7 times the size of the sun, it will become a white dwarf. White Dwarfs are small stars about the size of Earth, but they are hotter and were once the core of the progenitor star. Neutron Stars are the cores of original stars that have been crushed to the size of a city and they are packed up with Neutrons, the part of the atom that has no electrical charge. One 1 cm x 1 cm x 1 cm cube of their material would weigh as much as 5 fully-loaded cargo ships.

Luminosity Class

Stars are also organized into another classification, this time for luminosity. This tells if the star is illegible to be called a dwarf, giant, supergiant, or hypergiant. It is also called the Yerkes spectral classification.

Temperature Class

The stellar classification used to classify stars by color or temperature has seven main classes and four other ones. It is called the Morgan-Keenan stellar classification. It was originally proposed by Italian astronomer Angelo Secchi, but has been improved by Harvard professors William Morgan and Philip Keenan.

There are stellar classes P and Q, which are used for planetary nebulae and novae and pulsars respectively.

Objects included are the Sun, the red and brown dwarf companions of Gliese 229, Teide 1 and Jupiter.

Brown Dwarfs are stellar objects that cannot fuse hydrogen with their interiors, and thus can only radiate in infrared light. The first such object discovered was Teide 1, located in the constellation Taurus.

Hertzsprung-Russell Diagram

Star types are arranged in the Hertzsprung-Russell Diagram. This scattergraph was invented by Danish and American astronomers Ejnar Hertzsprung and Henry Russell in 1908. Stars farther away on the right are cooler in temperature, while stars that are near the top are more luminous and bright.

The line going from the top left-hand side down to the bottom right-hand side contains all of the main-sequence stars. The giant branch, containing yellow, orange, red giants, supergiants, hypergiants, and Mira variables, is located just above and right of the main sequence stars. Blue and white supergiants are directly above the central main sequence. Brown dwarfs are at the very bottom of the main sequence, with white dwarfs directly below the central main-sequence. The yellow evolutionary void is located between the blue-white supergiants and the giant branch.

B-V Color Index

A graph comparing several values of the B-V Color Index, with other possible but non-black body colors for comparison

In astronomy, the B-V color index is a numerical expression that determines the exact "color" of a certain star. The smaller the color index, the more the star appears blue. For comparison, our Sun has an approximate B-V color index of 0.656[21], while Rigel has a color index of -0.03.[22] Red Giants have color indexes from 0.81 to 1.4 .[23]

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